1. Field of the Invention
The invention relates to a process for processing mixed waste, in particular mixed waste consisting mainly of plastics, paper-plastics-composite materials, glass, metals, paper, cardboard, and other waste substances. Furthermore, the invention relates to a processin plant for performing the process.
2. Discussion of the Background
When the dual system was introduced with regard to implementing the packaging regulations in Germany, a special challenge proved to be the proper recycling of the collected plastic materials. In doing so, a completely new fraction, namely the mixed plastic materials, came onto the market. For the mixed plastics, because of their heterogeneity and fluctuating composition, special recycling methods must be found. For the recycling of mixed plastic materials, mainly a raw material recycling can be considered. A pre-requisite for this is a processing of the material into an agglomerate which meets predetermined specifications. Since the plastic materials are collected together with a number of other waste materials within the framework of the Dual System, processes are required which reduce the sorting work to an acceptable limit, but nevertheless ensure a certain degree of purity of the plastic material.
Metallic waste matter can be separated by means of magnetic separators and eddy current separators. Heavy plastic materials and other heavy foreign components are usually separated from the material to be processed by means of air sifting. Furthermore, various processes are known for removing paper particularly from paper-plastic composite materials.
With the improved separation of waste materials, nowadays dry processes can be used for processing mixed plastic materials with which a good quality plastic material agglomerate can be produced at a comparatively low energy consumption. In this connection the process disclosed in the WO 96/20819 is being widely used, with which the material to be processed is first comminuted and freed from magnetic substances. The comminuted material is then compacted in an agglomerator by pressing or it is thermally agglomerated, whereby volatile substances, such as, for example, water vapour, ash and paper, are drawn off by a suction device. Then the agglomerated material is dried and subsequently screened.
For economical reasons it is desirable to ensure a continuous operation of the plant. This is not possible with the process known from the WO 96/20819. The breakdown of one plant component usually results in the shut-down of the entire plant and leads to outage times caused by repair and maintenance work. A further disadvantage lies in the fact that the paper is not removed from the waste stream in notable quantities. It is apparent from the above said that paper is only removed as a volatile material using the process according to WO 96/20819, so that, for example, plastic-paper composite materials can only be partially digested or not at all. Thus, the agglomeratiel contains a considerable amount of paper.
U.S. Pat. No. 5,464,100 describes a sorting device for solid waste consisting of a storage station having a discharge section, several conveying sections being arranged one after the other, a sorting device, and a wind sifter. Therein, a second set of conveying belts is arranged immediately behind a first set of conveying belts, wherein the conveying speed is increased on the second set of conveying belts with respect to the first set. This is used to limit the stack depth of the waste on the second set of conveying belts during a subsequent manual sorting.
It is the object of the present invention to provide a process for processing mixed waste, which mainly consists of plastic materials, cardboard, paper, paper/plastic composite materials, glass, metals, and other waste substances, for providing plastic material being substantially free of foreign materials to an agglomerator.
The above-described object of the present invention is achieved by a process as described in the following paragraphs.
A process according to the invention for processing mixed waste for the operation of a continuously operating dry-mechanical processing plant, which comprises a plurality of successive processing stations each connected by at least one conveying section, with at least the steps in the sequence metal separation, air sifting and paper removal, between which further steps may be provided or which follow one another directly, is characterized in that the good-material flow, during the transfer to the last conveying section before the processing station experiences an acceleration therefrom, so that the average actual speed of the particles of the good-material flow on the last conveying section immediately before entering the processing station of the metal separation is slower than immediately before it enters the processing station of the wind sifter and this in turn is slower than the average speed immediately before it enters the processing station of the paper separation.
In this regard, the good-material flow is the material flow of qualitatively high-grade material, i.e. of the material which must still be further processed in the further course of the process, and the actual speed of the particles is the actual path travelled by the particles per time.
Preferably, the mixed waste flow between two adjoining stations is conveyed on the respective conveying section at a substantially constant speed.
It may furthermore be provided that in at least one of the stations a stock of possibly pre-treated material is temporarily stored.
An embodiment of the process according to the invention is characterized by the following steps:
(a) Comminuting of the mixed waste;
(b) Intermediate storage of the comminuted mixed waste;
(c) Uniform conveying of the intermediately stored mixed waste;
(d) Separating of magnetic metal particles;
(e) Separating of non-magnetic particles of materials with a specific weight which exceeds a predetermined minimum specific weight;
(f) Separating of paper, for example from the paper/plastic composite materials;
(g) Intermediate storage of the obtained plastic material fraction; and
(h) Agglomerating of the plastic material fraction.
The term agglomerating herein means the compacting into a bulk material by supplying heat and/or friction energy to the material to be agglomerated.
Optionally further process steps may be added, for example after the intermediate storage of the obtained plastic material fraction another separating of non-magnetic particles may take place. Furthermore it may be advantageous to comminute the agglomerated plastic material to a fixed particle size. Sometimes it may be expedient to subsequently carry out another separation of magnetic metal particles that only become accessible to a magnetic separation after the comminuting process.
According to another embodiment the process according to the invention is carried out in such a way that the material to be processes is conveyed only pneumatically on certain conveying sections.
The invention is based on the finding that an effective separation of the paper is only successful if the flow of the waste material to be treated is accelerated, i.e. practically a xe2x80x9cwidening upxe2x80x9d of the material takes place. This facilitates the separation of waste substances in the various waste separators.
A continuous process can easily be carried out when it is ensured that at a critical points of the processing process an intermediate storage of the material processed up to then takes place. As already mentioned at the outset, waste comminutors or shredders are such critical components. For this reason a processing plant usually comprises several such shredders, all of which operate into a buffer silo. Thus, the breakdown of one shredder does not result in a shutdown of the plant since the material from the buffer silo can still be fed to the following plant components. In this connection it is also possible to utilize the known advantage of a buffer silo in that it has a homogenizing effect on the pre-comminuted material and the same can then be passed on to the following plant components in a uniform composition. Shredders or waste comminutors pulsate very heavily during operation, which makes a uniform feeding of the following waste material separators difficult. The buffer silo, therefore, also acts as a mechanical decoupling from plant components.
Following the buffer silo, at least one magnetic separator may be provided, furthermore at least one device for separating non-magnetic particles from materials with a specific weight which exceeds a predetermined minimum specific weight. It has proved advantageous to use so-called gravity pipe sifters. With such gravity pipe sifters metallic and non-metallic heavy particles are filtered off, so that an eddy current separator can be usually dispensed. The plant furthermore comprises at least one device for separating paper, for example from the paper/plastic composite materials, which is/are followed by a buffer silo in which the plastic material fraction from each device for the separating of paper is collected. In this way it becomes possible to use a continuously operating disc compactor which can be obtained, for example, as CV 50 from the company Netztsch Condux, Hanau.
The homogenizing and evening-out effect of the buffer silos in which the plastic material fraction is collected can be utilized to connect a further device for separating heavy non-magnetic particles.
A buffer silo for a processing plant, being provided following the shredder or shredders, consists of a housing with at least one opening in the upper part of the housing for feeding in the material for the intermediate storage and at least one discharge opening for the material and is characterized in that in the bottom part of the housing a plurality of discharge screws are provided, wherein the discharge screws are arranged in such a way that their action covers the entire bottom area of the housing and at least one fore-runner screw is provided which homogenizes the material discharge from the discharge screws. This prevents that larger quantities of material rush into the chambers of the scraper chain conveyor.
Here the discharge screw may each operate with a different direction of rotation, e.g. they may optionally operate turning to the left or to the right.
Advantageously the discharge screw are arranged parallel to one another and the fore-runner screw is arranged offset by 90xc2x0 relative to the discharge screws. The fore-runner screw may also optionally operate turning to the left or right.
A buffer silo for a processing plant in which the plastic material fraction is collected, consists of a housing with at least one opening in the upper part of the housing for feeding in the material for the intermediate storage and at least one discharge opening for the material and is characterized in that at least one loosening-up screw placed under pressure is provided for the material stored in the buffer silo and that a suction device supplies air from the housing to the at least one loosening-up screw. Furthermore, special discharge screws are provided in the buffer silo, which convey the intermediately stored material to the at least one loosening-up screw.
It has proved expedient to widen the housing conically or in a trapezoidal shape towards the bottomxe2x80x94depending on the basic shape of the housingxe2x80x94to avoid a bridge formation in the silo.